1. Technical Field
The present invention generally relates to pneumatic tires and, more particularly, to a pneumatic tire having a multiple modulus body cord that improves the rolling resistance of the tire. Specifically, the present invention is directed to a pneumatic tire having a body cord divided into upper and lower sections wherein the ratio of the modulus of the upper section to the modulus of the lower section is reduced in order to decrease the rolling resistance of the tire. The ratio is reduced by providing additional body cords in the lower section or by increasing the modulus of the cords in the lower section.
2. Background Information
In an inflated and loaded condition, a radial tire is subject to bending moments at the shoulder areas in both the leading and trailing positions of the tire footprint. The strains and stresses created by the moments are directly related to the rolling resistance of the tire. Previous research and studies have focused on reducing the density of strain caused by the bending moments to reduce rolling resistance. One theory to reduce rolling resistance in a pneumatic tire is to increase the eccentricity (ECY) of the tire. Eccentricity is the ratio of belt tension (BT) over body cord tension (BCT). ECY thus equals BT/BCT. In general, a higher ECY will result in lower rolling resistance.
The art has also determined that ECY is related with fundamental stiffness and primarily with radial stiffness (Kr). Fundamental stiffness is a group of three parameters measured through a specially designed testing machine. The three parameters are lateral stiffness (Ks), radial stiffness (Kr), and tangential stiffness (Kt). The eccentric stiffness (Ecc) may be defined as Trr(Kr+Kt). ECY is inversely proportional to Ecc. A tire with a lower Kr will thus have a higher ECY and lower rolling resistance.
In the past, rolling resistance requirements have been met by changing the materials or design parameters of the tire. Most tire designs change the tread compound in order to meet the rolling resistance requirements. Changing the tread compound is generally undesirable and the art desires another method for reducing the rolling resistance in tire design.
In view of the foregoing, an aspect of the present invention is to reduce the rolling resistance of a pneumatic tire by improving the carcass design. In particular, the invention improves the rolling resistance by optimizing the body cord stiffness. The invention optimizes body cord stiffness by dividing the body cord in a tire section into three sections and optimizing the ratio of stiffness of the areas to reduce rolling resistance.
The invention provides a relationship between the body cord tension ratio and the radial stiffness and a relation between radial stiffness and the rolling resistance of the tire.
The invention also provides a pneumatic tire having a multiple modulus body cord tire by applying the concepts of the invention to reduce rolling resistance. The invention is thus achieved by a pneumatic tire including a body having maximum section width. The tire also includes an upper section above the maximum section width and a lower section below the maximum section width. The tire further includes a ply of body cords disposed in the body. The modulus of the body cord ply in the lower section is larger than the modulus of the body cord ply in the upper section.
The invention also provides a pneumatic tire including a body having maximum section width with an upper section above the maximum section width and a lower section below the maximum section width. The tire also includes a plurality of body cords disposed in the body. Each of the body cords having an upper section and a lower section wherein the lower section has a larger modulus than the upper section.
The invention further provides a pneumatic tire including a body having maximum section width with an upper section above the maximum section width and a lower section below the maximum section width. A plurality of body cords are disposed in the body. An intermediate cord is disposed between each pair of body cords in the lower section of the body.